This invention generally relates to cutting tools for machining bores in workpieces, and is specifically concerned with drills and reamers for machining bores in metal materials having a relatively high degree of spring-back or memory without the occurrence of sticking or galling between the inner walls of the bore and the outer diameter of the tool.
Cutting tools such as drills or reamers for machining bores in metal workpieces are well known in the prior art. Prior art drills typically include a shaft that terminates in a drill point formed from two or more cutting blades which are radially oriented with respect to the shaft axis of rotation. The cutting blades are separated by helical flutes for removing chips and swarf generated as the cutting blades of the drill point penetrate the material to form a bore in a workpiece. The fluted portion of the drill terminates in a cylindrical or tapered shank that is gripped by a chuck that is turned by a motor. In order to insure that the outer diameter of the fluted portion of the drill does not rub against a bore being created by the drill point, it is known to slightly taper the fluted portion of such drills a few thousandths of an inch so that the diameter of the drill diminishes between the drill point and the drill shank.
Reamers are used to size and finish preexisting bores. To this end, they employ a plurality of straight, longitudinally oriented cutting blades disposed around the circumference of the tool shaft. The cutting blades are separated by straight flutes which receive and remove the metal chips and swarf generated by the reaming operation. Unlike drills, such prior art reamers are not tapered from their leading to their trailing ends, but maintain the same diameter throughout the longitudinal lengths of the cutting blades. The modest material removed during a reaming operation, in combination with the burnishability and small amount of spring-back associated with most metals, normally allows such reamers to perform their sizing and finishing function without undue friction or sticking.
While such prior art boring and reaming tools generally function well when machining workpieces formed from metals having relatively little spring-back (i.e., steel), the applicants have observed that problems arise when such tools are used to machine deep bores in metal alloys having large spring-back properties such as aluminum, titanium, and nickel alloys. Specifically, when a prior art drill is used to machine a deep bore in a workpiece formed from such an alloy, the inner diameter of the bore created by the drill point can actually spring back or radially contract a few thousandths of an inch behind the tip of the drill as it penetrates the workpiece, thereby ultimately bringing the inner walls of the bore into contact with the outer fluted surface of the drill. The heat created by the resulting frictional contact can not only cause the drill to seize up inside of the bore (and in some instances break) due to sticking and galling, but can sometimes, cause the workpiece to ignite when it is formed from a flammable metal such as titanium. Prior art reamers used in high spring-back materials can also generate undesirable amounts of friction during the reaming operation. Normally, the leading tip of the reamer does most of the work, widening the bore so that the tips of the reamer blades have only light contact with the wall of the bore. However, when such reamers are used to machine workpieces formed from high spring-back alloys, the consequent radial contraction of the bore behind the leading tip of the reamer can cause the tool to seize up and break due to the localized melting and subsequent galling that occurs between the outer tips of the reamer blades and the inner surface of the bore.
Clearly, there is a need for cutting tools for machining bores in workpieces formed from alloys having high spring-back characteristics. Ideally, such cutting tools would be capable of either forming or reaming a bore in a workpiece formed from such a material with little or no contact between the sides of the tool and the inner surface of the bore so as to eliminate any possibility of galling, seizing, and breakage. It would be desirable if such a tool had a relatively simple design which could be incorporated into prior art drills and reamers. Finally, such a cutting tool should have a long tool life, and be equally effective in boring or reaming materials formed from alloys having high or low spring-back characteristics.